The present invention relates to the field of optics, in particular, to optical devices for spatially separating or combining orthogonally polarized optical beams, in particular, to optical devices used as optical beam polarizers or analyzers in the optics of ultraviolet, visible, and infrared radiation, including laser emission. More specifically, the invention relates to birefringent polarizing two-beam prisms.
According to commonly accepted rule, orientation of the light-wave electric field determines its polarization direction, and the plane of the electric vector and the light propagation direction are referred to as a polarization plane. If electric field oscillations occur only in that plane, and the plane itself has a constant spatial position, such light is referred to as having linear or planar polarization (or simply polarized). If the wave electric vector rotates around the light propagation direction (i.e., around the wave vector), such light can have either elliptical or circular polarization. For nontonochromatic light, i.e., for one containing a number of frequency components, the temporal changes in the amplitude and spatial position of its resulting electric vector can be absolutely arbitrary, and such light is referred to as unpolarized.
Linearly polarized light beams have found general application in optics, laser engineering, technology, e.g., for precision processing of metals (cutting, drilling etc.), in photochemistry for resonance excitation of molecules and atoms, in biology for similar purposes, in communication engineering, etc. The preference is given to polarized light due to higher accuracy of interaction of such light with materials. Such high interaction accuracy results from the complexity and anisotropy in the inner structure of the aforementioned materials. For example, most of the devices widely used in optics and communication engineering for entering information into a light beam, such as electrooptical and acoustooptical modulators, operate with linearly polarized light because of the pronounced anisotropy of optical properties in the crystals these devices are based upon. Fiber optics communication engineering is a field where polarized light has a constantly increasing application. Anisotropic fibers for polarized light and low-noise polarization amplifiers have been developed. In principle, polarized radiation is used for effective transformation of laser frequencies in nonlinear crystals and for selection of optical radiation frequencies by anisotropic tunable acoustooptical and electrooptical filters. The use of polarized light is required for operation of binary polarization switchers/modulators, polarization multiplexers and, in general, in any optical devices for which anisotropic interaction of light with the materials is advantageous.
There are a number of devices that can be used for light polarization. These include dichroism dye based polarizers, purely crystalline polarizers, interference polarizers, polarizers based on isotropic materials that use the effects of light reflection and light refraction at the Brewster angle, etc.
However, special accent is made on prism-type polarizers that have a specific geometry and are made of optically anisotropic crystalline materials. The reason for making such accent is due to the special properties of these polarizers. As a rule, they are crystalline polarizers that exhibit high extinction (ratio of the useful and unnecessary orthogonally polarized light components) of polarized beams, low optical losses, and high resistance to high-power optical radiation, especially laser radiation.
For better understanding the principles of the present invention, it would be advantageous to briefly describe the structure of conventional polarizing prisms. The basics of polarizing devices are described, for example, in Handbook of Optics, Vol. II, Devices Measurements and Properties, McGraw-Hill, Inc., 1995, pp. 3.1-3.70, New York, San Francisco, Montreal, Tokyo, Toronto.
Polarizing prisms are made only of birefringent crystals that have no cubic crystal symmetry. In such crystals, the light is split into two orthogonally polarized beams which, upon exit from the crystal, are in general case spatially separated both with respect to the exit points and the propagation angles. However, for many reasons (small separation angles or distances, unavoidable frequency dispersion of the prism, reflection optical losses and technologically uncomfortable beam exiting geometry) simple single crystal prisms are replaced for combinations thereof that are referred to as polarizing prisms. Polarizing prisms are usually made of a relatively cheap and abundant calcite (CaCO3). Recently a wide range of artificially grown birefringent crystals have been developed for polarizing prism applications. Such crystals are, for example, TiO2, YVO4, KNbO3, KTiOPO4, xcex1-BaB2O4, PbMoO4, TeO2, Te, Se, etc. However, the general use of these materials is precluded by their high cost, complexity of manufacturing compound prisms therefrom or insufficiently pronounced optical anisotropy (birefringence).
Advanced polarizing prisms usually contain two or more trihedral prisms made of optically uniaxial crystals of tetragonal, hexagonal, or trigonal symmetry having similar or different optical axis orientations and bonded to each other with transparent substances (cements) or separated from each other with a thin air or vacuum gap. Cement-free gaps are often used in prisms for short-wave radiation or high-power laser beams.
Prisms are subdivided into one-beam prisms, from which only one linearly polarized light beam exits, and two-beam prisms, that produce two light beams polarized in mutually perpendicular planes (orthogonally polarized beams). The former type prisms operate on the basis of the total internal reflection principle. A nonpolarized incident beam is split in the prism into two orthogonally polarized beams. One of these beams undergoes total internal reflection at the prism bounding and is deflected, while the other beam passes through the bounding for further use or processing. Such prisms are known as the Nicol, Glazebrook, Hartnack-Prazmowsky, Ahrens, etc., prisms. FIG. 1 shows some of these prisms. (a), (b) and (c) are Glan-type prisms known as the Glan-Thompson (a), Lippich (b) and Frank-Ritter (c) prisms. The second row in FIG. 1 shows Nicol-type prisms, i.e., the conventional Nicol prism (d), the Nicol-Halle form prism (e), and the Hartnack-Prazmowsky prism (f). The optical axes of the prisms are shown in with double arrows.
Variations in the structure of the prisms is normally accompanied by changes in the prisms"" names. For example, the air-gap Glan-Thompson prisms are referred to as the Glan-Foucault prisms, and the air-gap Lippich prisms as the Glan-Taylor prisms. In practice, any of these prisms can be referred to as a Glan prism. The air-gap Nicol prisms are referred to as the Foucault prisms. There also are combinations of three bound prisms, the so-called xe2x80x9cdoublexe2x80x9d prisms. The double Glan-Thompson prisms are referred to as the Ahrens prisms.
FIG. 2 shows various types of two-beam polarizing prisms. The optical axes of the two parts of Rochon (a), Sxc3xanarmont (b), and Wollaston (c) prisms are perpendicular to each other. The Foster (d) and beam-splitting Glan-Thompson (e) prisms have parallel optical axes. In this respect these prisms are similar to one-beam polarizing prisms, but their shape is changed so the two beams propagate in specific directions without noticeable losses.
The need for the great variety of existing polarizing prisms (not all of them are shown here) stems from the impossibility of designing a prism having universal parameters. Each polarizing prism has its individual advantages and drawbacks that determine its applicability. Prisms are characterized by a number of parameters, such as angular separation of the beams and frequency dispersion of the aforementioned angular separation, angular aperture, extinction, spectral operation range, optical losses, resistance to high-power optical radiation and external thermal, humidity and mechanical impacts, entrance hole (geometrical aperture), linear sizes, durability, manufacturability and, of course, cost.
Example of one of the latest polarizing prism beam splitter is given in U.S. Pat. No. 6,018,048 issued on Jan. 25, 2000 to J. Pan et al. This splitter consists of a collimator and two similarly shaped birefringent crystal prisms. The light from the collimator is incident upon the first face of the first birefringent crystal prism, which also has second and third faces. In the first prism the collimated light that has passed along a normal to the first entrance face is incident onto the second face at a certain angle. The light component polarized perpendicular to the incidence plane is reflected without losses from the second face and is directed towards the third face of the first prism, while the light component polarized in the incidence plane is refracted to the second prism through the gap between the prisms. This thin gap is formed by parallel second faces of the prisms. The first (exit) face of the second prism is positioned relative to the second face of the second prism in exactly the same manner as the first face of the first prism is positioned relative to the second face of the first prism. As a result, the light that exits the second prism is refracted essentially along a normal to the first face of the second prism without cross-sectional distortions.
It is noteworthy that in all aforementioned combined polarizing prisms, including the one described in U.S. Pat. No. 6,018,418, separation of polarized beams occurs on the boundary between the two optical elements. This is important because, apart from beam splitting, optical losses occur due to fundamentally unavoidable Fresnel reflection and the cement material absorption on the boundary. As has been noted, the optical losses put limits upon the applicability of prisms in the UV range and high-power coherent laser engineering because the cement layer in the gap between the optical elements is frequently destroyed by such radiation. Vacuum and air gap prisms are used in the above applications, but in that case Fresnel losses increase due to the removal of immersion on the gap boundaries, thus the applicability of this design is limited. This problem could be solved by using very thin gaps with thicknesses on the order of wavelength, but in that case, apart from serious technological difficulties, optical losses in the reflected beam would grow unavoidably. This will occur due to the penetration/tunneling of this beam through the gap, which effect would unavoidably impair the forward beam extinction ratio. Depending on prism design, such losses may be as high as 10%.
Another disadvantage of multicomponent prisms is their complex and troublesome technology. In their manufacturing it is necessary to provide high optical quality on cemented surfaces, exact mutual orientation of the crystal prisms, high-quality cementing without inclusions, and uniform gap thickness. It is also necessary to take into account anisotropic thermal expansion in the prism components, especially in case of different optical axis orientations, choose an appropriate cementing composition, etc.
An essential disadvantage of all known polarizing prism, including the one described in U.S. Pat. No. 6,018,418, is that they have unavoidable dispersion in the angular special positions of at least one of the polarized beams. This is because in composite prisms the separation of polarized beams into two separate beams occurs on the boundary of prism components at relatively large incident angles. Such separation is always accompanied by variations which occur in a beam separation angle between the polarized beams and which depends on the light wavelength.
The applicant has developed an anizotropic single-crystal polarizing prism for separation of a non-polarized beam into two orthogonally-polarized beams with minimal optical losses. This prism is described in detail in my earlier pending U.S. patent application Ser. No. 09/844,906 filed on Apr. 27, 2001 and its disclosure is incorporated by reference herein in its entirety. The aforementioned prism has a tetrahedral shape and is formed by four base planes having a predetermined angular orientation with respect to each other so that one of the polarized beams exists the prism at a Brewster angle by being refracted on the third base plane, while the second beam is reflected from the third base plane with total internal reflection and exits the prism through the fourth base plane in the normal direction thereto.
The prism of U.S. patent application Ser. No. 09/844,906 is shown in FIG. 3, which is a longitudinal sectional view of the prism with illustration of path of the beams. This prism, which in general is designated by reference numeral 56, is made of a uniaxial or a biaxial crystal. Let us assume, for simplification of description, that it is made of a uniaxial crystal, such as calcite (CaCO3). As can be seen from FIG. 3, the prism comprises a four-sided polygon and has four base planes, i.e., a first base plane 58, a second base plane 60 forming an angle xcex1 with the first base plane 58, a third base plane 62 located opposite to the second base plane 60 and forming an angle xcex2 with the first base plane 58, and a fourth base plane 64 located opposite to the first base plane 58 and forming an angle xcex3 with the second base plane 60. It is important to note that all four angles of the prism differ from each other and all base planes, i.e., all side of the four-sided polygon, are not parallel. At least the first base plane 58, through which the non-polarized beam 50 enter the prism, and the fourth base plane 64, through which one of the polarized beams (which is described later) leaves the prism, are coated with antireflective interference coatings 51 and 53 shown on FIG. 3.
The entrance beam 50 enters the prism 56 of FIG. 3 through the first base plane 58 along the normal to this plane. The incident beam 50 reaches the second base plane 60 and is reflected therefrom towards the third base plane 62 in the form of two orthogonally-polarized beams, i.e., an e-beam 52 and an o-beam 54. In order to provide minimal optical losses, in the prism 56 of the type described in the aforementioned patent application, the e-beam 52 exits the prism 56 through the third base plane 62 with the Brewster refraction. At the same time, the o-beam 54 is reflected from the third base plane 62 with full internal reflection, which also is a criterion required for minimization of optical losses, and exits the prism 56 via the fourth base plane 64 along the normal to this plane.
It should be noted that the prism 56 of the type described in the aforementioned U.S. patent application Ser. No. 09/844,906 was developed for the purpose of obtaining a single-crystal polarizing beam splitter that provides a large beam separation angle with minimization of optical losses without the use of multicomponent optical devices such as composite prisms. This objective was successfully accomplished. Furthermore, it has been found that along with reduced optical losses, the aforementioned prism is characterized by reduced wavelength dispersion of the beam separation angle in air. In FIG. 3 this angle is shown as angle xcex94. The aforementioned reduction in the wavelength dispersion is achieved due to the fact that dispersions of the reflection angle of the e-beam and of the Brewster refraction angle in air are opposite in signs and therefore are mutually subtracted. The resulting dispersion can even be reduced to zero by adjusting the angle of incidence and deviation from the Brewster angle. However, the such a decrease in dispersion is achieved at the expense of increase in optical losses.
Another disadvantage of the prism 56 is a small anamorphism inherent in e-beam in air at arbitrary choice of the beam incidence angles. In other words, the beam is slightly compressed in the incidence/refraction plane.
It is an object of the invention is to provide a single-crystal two-beam polarization prism which is free of wavelength dispersion of the beam separation angle in air. Another object is to provide a prism of the aforementioned type through which the beams pass without distortion if their cross sections. Another object is to provide a prism of the aforementioned type which combines the aforementioned features with minimal optical losses. Another object is to provide a method of manufacturing the aforementioned a single-crystal two-beam polarization prism of the invention.